The ACIA is illustrated in figure 3. I am using this ACIA because it is much easier to understand than newer serial interfaces. Once you understand how the . MC Asynchronous Communications Interface Adapter (ACIA) F8DCh CPCI Serial Interface MC Control/Status Register (R/W). Computers transfer data in two ways. Parallel. Serial. Parallel data transfers often 8 or more lines are used to transfer data to a device that is only a few feet away.

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Source file VHDL/ACIA_6850.vhd

At the receiving end of an asynchronous serial data link, the receiver continually monitors the line looking for a start bit. An asynchronous serial data link is character orientedbecause information is transmitted in the form of groups of bits called characters.

The nature of these signals is strongly affected by one particular role of the ACIA, its role as an interface between a computer and the public switched telephone network via a modem. The term break originates from the old current- loop data transmission system when a break was affected by disrupting i. The software necessary to drive the ACIA in this minimal mode consists of three subroutines: This operation can even be performed dynamically, if the need ever arises.

Many modern ACIAs include on- chip receiver and transmitter clocks, relieving the system designer of the necessity of providing an additional external oscillator. The baud rate generator is bypassed when the device is used in the divide by 1 mode.

6850 ACIA chip

It is also possible to operate the ACIA in a minimal interrupt- driven mode. Table 7 provides a simplified extract from the DUART’s data sheet that describes the five control registers. That is, the ACIA contains almost all the logic necessary to provide an asynchronous data link between a acai and an external system.

When the received character has been assembled, its parity is calculated and compared with the received parity bit following the character. The called a DUART performs the same basic functions as a pair of s plus a baud- rate generator. The five control registers are: This bit is cleared either by loading the transmit data register or by performing a software reset. Until the introduction of USB the most popular serial interface between a computer and its CRT terminal is the asynchronous serial interface.


These bits select also the type of parity if any and the number of stop bits. Suppose you have a program with a bug that executes an unintended write to a write- only register. Afterwards, a secondary reset can be performed by software, as we shall describe later.

ACIA chip – CPCWiki

The Asynchronous Serial Interface The vast majority of general- purpose microcomputers, except some entirely self- contained portable models, once used a serial interface to communicate with remote peripherals such as CRT terminals. It is not possible to provide a full input routine here, as such a routine acua include recovery procedures from the errors detected by the ACIA.

Baud Rate Generator The crystal oscillator feeds a programmable baud rate generatorthat is capable of generating 1 of 7 baud rates for a single crystal.

The key to the operation of this type of link is both simple and ingenious. This material is taken from articles I wrote on the 68K microprocessor. Finally, the transmitter sends a stop bit at a mark level i. The purpose of this exercise is two- fold. Each routine tests the appropriate status bit and then reads data from or writes data to the ACIA’s data register. The transmitted data from the computer becomes the received data at the CRT terminal.

The clocks operate at 1, 16, or 64 times the data rate. Some of the output functions that can be selected are: For example, the OR instruction would read the contents of the ACIA’s status register, perform a logical OR and then write the result back to its control register.

I am using this ACIA because it is much easier to understand than newer serial interfaces. You cannot detect the change by reading back the contents of the register. I am perfectly happy to accept read- only registers, but I am suspicious of the write- only variety because it is impossible to verify the contents of a write- only register. Note that CR7 is a composite interrupt enable bit and enables all the three forms of receiver interrupt described above.

It is there for the purpose of compatibility with older equipment. As each incoming bit is sampled, it is used to construct a new character. Incoming and outgoing are used with respect to the ACIA. In a minimal, non- interrupt mode, bits 2 to 7 of the status register can be ignored. The two items at the computer end of the data link enclosed in clouds in figure 1 represent the software components of the data link.


In fact, the asynchronous serial data link is a very old form of data transmission system and has its origin in the era of the teleprinter. This is a perfectly logical, indeed an elegant, thing to do. To clear SR2, the CPU must read the contents of the status register and then the contents of the data register.

They are included to. Finally, we look at a more modern aia performance serial interface. This function is often performed by a single device called an asynchronous communications interface adaptor ACIA.

This status bit is set at the midpoint of the last bit of the second character received in succession without a read of the RDR having occurred. Data- carrier- detect status bit SR2 set and receiver interrupt enabled.

The ACIA has an internal baud rate generator. The latter mode results if the internal baud rate generator is. One of the first general- purpose interface devices produced by semiconductor manufacturers was the asynchronous communications interface adaptoror ACIA. Table 2 shows how the eight bits of the control acix are grouped into four logical fields.

It is so called because the transmitted data and the received data are not synchronized over any extended period and therefore no special means of synchronizing the clocks at the transmitter and receiver is necessary.

Transmitter data register empty SR1 set and transmitter interrupt enabled. The RDRF bit is cleared either by reading the data in the receiver data register or by carrying out a software reset on the control register. The framing error status bit is automatically cleared or set during the receiver data transfer time and is present throughout the time that the associated character is available.

This condition may be employed to force an interrupt at a distant receiver, because the asynchronous serial format precludes adia existence of a space level for longer than about ten bit periods.